1. Field of the Invention
The present invention relates to a skew-correcting device that corrects skew of a sheet, and a sheet-processing apparatus that processes the sheet with the skew thereof corrected by the skew-correcting device.
2. Description of the Related Art
An example of a sheet-processing apparatus includes a puncher that punches a sheet having an image formed by an apparatus body of an image forming apparatus (see Japanese Patent Laid-Open No. 10-194557, corresponding to U.S. Pat. No. 5,911,414)). The image forming apparatus may be a copier, a printer, a facsimile, or a multi function peripheral.
The puncher punches the sheet after the skew of the sheet is corrected by the skew-correcting device. The skew correction of the sheet means that the sheet inclined relative to a sheet-conveying direction is corrected to be along the sheet-conveying direction.
FIG. 12 is an illustration showing a puncher 410. The puncher 410 is disposed in a sheet-conveying path 430. A sheet P is punched using a punch 411, which is linearly movable in a reciprocating manner in a direction indicated by an arrow A, and a fixed die 412, after the skew of the sheet P is corrected by a skew-correcting device 401. The puncher 410 punches the sheet P by moving in a reciprocating manner.
The skew of the sheet is corrected as follows. The sheet P conveyed by an upstream conveying roller pair 406 is continuously conveyed by a conveying roller pair 405. When the rear end (lower end in the drawing, i.e., the upstream end in the conveying direction) of the sheet P passes through a rear-end stopper 421, the rear-end stopper 421 is rotated from a standby position plotted by the broken line to a receiving position plotted by the solid line. Then, the conveying roller pair 405 rotated forward in a direction indicated by an arrow B is rotated backward in a direction indicated by an arrow C, to feed the sheet backward. The rear-end stopper 421 at the receiving position receives the rear end of the sheet. This corrects the skew of the sheet at the rear end side thereof.
After the skew at the rear end side of the sheet is corrected, the puncher 410 makes holes at a portion of the rear end side.
The skew-correcting device 401 shown in FIG. 12 corrects the skew of the sheet by conveying the sheet in the direction opposite to the conveying direction once, so that the rear-end stopper 421 comes into contact with the rear end of the sheet. Therefore, the conveyance of the sheet is intermitted for the skew correction. This degrades the efficiency of the skew correction of the skew-correcting device 401.
Another example of the mechanism for correcting the skew of the sheet uses a shutter as disclosed in Japanese Patent Laid-Open No. 2005-154073. The skew of the sheet is corrected as a front end of the sheet conveyed by a sheet-conveying unit comes into contact with the shutter. The front end of the sheet is corrected by the shutter, and the sheet is conveyed while the sheet pushes and moves the shutter. In other words, the shutter is pushed and rotated by the sheet to be conveyed. With this mechanism, the hardness of the sheet to be conveyed by the sheet-conveying unit is utilized for moving the shutter against a force, for example, caused by a spring that biases the shutter. If a sheet with a small degree of hardness can move the shutter, a sheet with a large degree of hardness possibly moves the shutter even before the skew of the sheet is reliably corrected. Thus, it is difficult to correspond to various types of sheets having different levels of hardness while providing highly accurate skew correction.